The binding domain structure of retinoblastoma-binding proteins

The retinoblastoma gene product (Rb), a cellular growth suppressor, complexes with viral and cellular proteins that contain a specific binding domain incorporating three invariant residues: Leu-X-Cys-X-Glu, where X denotes a nonconserved residue. Hydrophobic and electrostatic properties are strongly conserved in this segment even though the nonconserved amino acids vary considerably from one Rb-binding protein to another. In this report, we present a diagnostic computer pattern for a high-affinity Rb-binding domain featuring the three conserved residues as well as the conserved physico-chemical properties. Although the pattern encompasses only 10 residues (with only 4 of these explicitly defined), it exhibits 100% sensitivity and 99.95% specificity in database searches. This implies that a certain pattern of structural and physico-chemical properties encoded by this short sequence is sufficient to govern specific Rb binding. We also present evidence that the secondary structural conformation through this region is important for effective Rb binding.

Thrombin-induced expression of endothelial P-selectin and intercellular adhesion molecule-1: a mechanism for stabilizing neutrophil adhesion

Thrombin-induced expression of endothelial adhesivity toward neutrophils (PMN) was studied using human umbilical vein endothelial cells (HUVEC). HUVEC were challenged with human alpha-thrombin for varying durations up to 120 min, after which the cells were fixed with 1% paraformaldehyde and 51Cr-labeled human PMN were added to determine PMN adhesion. Endothelial adhesivity increased within 15 min after alpha-thrombin exposure, and the response persisted up to 120 min. Expression of endothelial adhesion proteins, P-selectin (GMP-140, PADGEM, CD62), and intercellular adhesion molecule-1 (ICAM-1; CD54) on the endothelial surface was quantitated by increase in the specific binding of anti-P-selectin mAb G1 and anti-ICAM-1 mAb RR1/1 labeled with 125I. P-selectin expression was maximal at 5-15 min alpha-thrombin exposure and decayed to basal levels within 90 min. In contrast, ICAM-1 activity increased at 30 min and remained elevated for 120 min after alpha-thrombin challenge. The initial endothelial adhesivity was dependent on P-selectin expression since PMN adhesion occurring within the first 30 min after alpha-thrombin challenge was inhibited by mAb G1. The later prolonged PMN adhesion was ICAM-1 dependent since this response was inhibited by mAb RR1/1 and to the same degree by the anti-CD18 mAb IB4. Anti-ELAM-1 mAb BB11 had no effect on adhesion of PMN to the alpha-thrombin-challenged cells. The initial P-selectin expression and PMN adhesion responses were reproduced by the 14-amino peptide (SFLLRNPNDKYEPF) (thrombin-receptor activity peptide; TRP-14) which comprised the NH2 terminus created by thrombin's proteolytic action on its receptors. However, TRP-14-induced PMN adhesion was transient, and TRP-14 did not cause ICAM-1 expression. The ICAM-1-dependent PMN adhesion mediated by alpha-thrombin was protein synthesis independent since ICAM-1 expression and PMN adhesion were not inhibited by cycloheximide pretreatment of HUVEC. Moreover, Northern blot analysis indicated absence of ICAM-1 mRNA signal up to 180 min after alpha-thrombin challenge. In conclusion, thrombin-induced endothelial adhesivity involves early- and late-phase responses. The initial reversible PMN adhesion is mediated by rapid P-selectin expression via TRP-14 generation. Thrombin-induced PMN adhesion is stabilized by a protein synthesis-independent upregulation of the constitutive ICAM-1 activity which enables the interaction of ICAM-1 with the CD18 beta 2 integrin on PMN.

Thrombin receptor 14-amino acid peptide binds to endothelial cells and stimulates calcium transients

We examined the binding characteristics of the recently described thrombin receptor amino-terminal peptide, SFLLRNPNDKYEPF (T. K. H. Vu, D. T. Hung, V. I. Wheaton, and S. R. Coughlin. Cell 64: 1057-1068, 1991), termed TRP-14, and its effect in activating intracellular calcium transients in pulmonary vascular endothelial cells. Binding of 125I-labeled TRP-14 was found to be saturable with a affinity constant of 2 microM and maximum binding of 41 pmol/mg of cell protein. The 125I-labeled TRP-14 also interacted with bovine pulmonary microvessel endothelial cells, human umbilical vein endothelial cells, and porcine pulmonary artery smooth muscle cells. Binding of 125I-labeled diisopropylphosphoryl (DIP)-alpha-thrombin, which is catalytically inactive but binds to thrombin receptors, was not inhibited by TRP-14 or vice versa, indicating that TRP-14 did not compete for the alpha-thrombin binding site(s) on the endothelial cell surface. TRP-14 (> 1 microM) increased the concentration of intracellular calcium ([Ca2+]i) in endothelial cells with kinetics similar to the increase in [Ca2+]i triggered by alpha-thrombin. In contrast, DIP-alpha-thrombin did not increase [Ca2+]i and also did not prevent the rise in [Ca2+]i induced by the subsequent challenge with either TRP-14 or alpha-thrombin. Because the generation of TRP-14 by the proteolytically active forms of thrombin stimulated a rise in endothelial [Ca2+]i, TRP-14 may be the agonist responsible for the activation of the alpha-thrombin receptor in pulmonary vascular endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a subunit contact site of the alpha-subunit of follitropin

Using synthetic peptides, we previously identified portions of the lutropin alpha-subunit that are in contact with the beta-subunit. In order to elucidate structure/function differences of the glycoprotein hormones, a similar study was conducted for follitropin. Peptides corresponding to the follitropin alpha-subunit were synthesized using standard solid-phase procedures. Purified peptides were incubated in the presence of alpha- and beta-subunits of follitropin, and subunit recombination was monitored using gel permeation chromatography and reverse-phase high-pressure liquid chromatography. Peptide alpha 33-58, corresponding to a highly conserved portion of alpha-subunit, completely inhibited subunit recombination for 24 h, and allowed only partial (61%-71%) recombination after 48 h. Peptide alpha 51-65 or alpha 61-78 inhibited subunit recombination partially at 24 h, but almost full (greater than 80%) recombination was observed by 48 h. Peptides corresponding to the rest of the alpha-subunit, alpha 1-15, alpha 11-27, alpha 22-39, and alpha 73-92, did not inhibit recombination of the alpha- and beta-subunits. The data suggest that alpha-subunits have similar residues in contact with regions of the beta-subunits of both lutropin and follitropin, specifically involving residues from continuous regions of the alpha-subunit (residues 45-75). The data suggest that this region contains multiple sites of contact with the beta-subunit.

Acetylated endothelin-1 is a constrictor in guinea pig lung vasculature but not in isolated vascular strips

We examined the relative importance of the two amino groups of endothelin-1 in mediating pulmonary vasoconstrictor activity. Complete acetylation of prefolded endothelin-1 (fAcET-1[AcK9]) yielded a product with vasoconstrictor properties (EC50 = 0.52 +/- 0.04 nM) in isolated Ringer-perfused guinea pig lungs similar to native endothelin-1 (EC50 = 0.31 +/- 0.05 nM). However, fAcET-1[AcK9] exhibited a marked reduction in potency when assessed by contraction of isolated guinea pig pulmonary artery strips or by contraction of carotid artery or aortic strip preparations. fAcET-1[AcK9] at concentrations up to 100 nM failed to induce appreciable contraction of any vascular strip preparation. In contrast, endothelin-1 had an EC50 of 1.46 +/- 0.32 to 1.88 +/- 0.19 nM in various vessel preparations. The differences in response to fAcET-1[AcK9] in the intact lung vs. strip preparation suggest different receptor populations in the two preparations. The importance of specific amino groups for contractile activity in the vascular strip preparation was explored by acetylation of individual sites (amino terminus or lysine sidechain) or both sites during peptide synthesis to produce AcET-1, ET-1[AcK9], and AcET-1[AcK9], respectively. The order of potency was endothelin-1 much greater than ET-1[AcK9] greater than AcET-1 greater than AcET-1[AcK9]. These results suggest that chemical modifications (e.g., biotinylation) should be made preferentially at the lysine-9 sidechain in order to retain maximal biological activity in vascular strip preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of endothelin receptor populations using endothelin-1 biotinylated at lysine-9 sidechain

Optimal conditions for biotinylation of endothelin-1 (Et-1) were determined using biotinylating reagents of variable linker arm length and mono- vs dual-biotinylated Et-1. Specific modification of lysine-9 sidechain with NHS-LC-biotin (Et-1[BtK9]) produced a derivative with maximal binding and retention of vascular smooth muscle contractile activity. The Et-1[BtK9] probe bound to Chinese hamster ovary cells transfected with EtA receptor cDNA (CHO[EtR]), but not untransfected cells. Binding to rat vascular smooth muscle cells (VSMC) was detectable at 0.01 nM with maximal binding at 1 nM. Displacement of 1 nM Et-1 [BtK9] binding by Et-1 indicated an IC50 value of 6 +/- 3 nM. Et-1 displaced Et-1[BtK9] binding to VSMC and CHO[EtR] to a greater extent than endothelin-3, indicating predominant expression of EtA receptor sub-type. Thus, biotinylation of Et-1 at the lysine-9 sidechain may be of general use for localization and typing of Et-receptor populations.

Design and synthesis of a specific endothelin 1 antagonist: effects on pulmonary vasoconstriction

The 21-amino acid vasoconstrictor peptide endothelin (Et) contains two disulfide bonds. We investigated the importance of the outer disulfide bond in Et-1 by replacing it with an amide linkage. Bioactivity was assessed in an isolated guinea pig lung preparation (perfused at constant flow with Ringer's solution/0.5% albumin) in which pulmonary artery pressure was monitored. Et-1 produced concentration-dependent pulmonary vasoconstriction at concentrations of 1 x 10(-10) M and higher. [Dpr1, Asp15]Et-1 (where Dpr is diaminopropionic acid), in which the outer disulfide was replaced by an amide bond and the inner disulfide was left intact, showed no agonist activity at 1 x 10(-6) M but 1 x 10(-7) M [Dpr1, Asp15]Et-1 inhibited Et-1-induced pulmonary vasoconstriction: effects of 1 x 10(-10) M 2 x 10(-10) M, and 1 x 10(-9) M Et-1 were inhibited by 98%, 75%, and 65%, respectively. Furthermore, this analog did not alter pulmonary vasoconstriction induced by thrombin, norepinephrine, or, most significantly, Et-3. A monocyclic Et-1 analog with the same sequence but in which the amide bond was not formed showed weak pulmonary vasoconstrictor activity (300-500 times less potent than Et-1) but had no antagonist activity. In addition, both the monocyclic control peptide and [Dpr1, Asp15]Et-1 competed effectively with 125I-labeled Et-1 for binding to cultured rat pulmonary artery smooth muscle cells. Thus, an Et-1 structural analog produced by replacement of the outer disulfide bond with an amide linkage displayed potent and specific Et-1 antagonism.

Structural characterization of a 14-residue peptide ligand of the retinoblastoma protein: comparison with a nonbinding analog

The retinoblastoma protein, a 110-kDa nuclear anti-oncoprotein, complexes specifically with transforming proteins of several oncogenic DNA viruses. A peptide [NLFCSEEMPSSDDE] derived from one of the viral proteins (simian virus 40 large T antigen) is known to competitively bind retinoblastoma protein, but a mutant analog [NLFCSKEMPSSDDE] does not. We studied the T peptide with HPLC to determine whether it can dimerize, and we employed circular dichroism spectroscopy to determine whether both peptides can exist in stable secondary structural conformations. HPLC analyses revealed that the T peptide is subject to oxidation and readily dimerizes. Circular dichroism analyses showed that both peptides can be induced to form stable secondary structural conformations under conditions that stabilize intramolecular hydrogen bonding in short peptides (90% 2,2,2-trifluoroethanol; 4 degrees C). The circular dichroism spectra of both peptide species were similar except for a statistically significant difference in the contour near 210 nm. Spectral analysis of the T-derived peptide species predicted elements of alpha-helix (18%), antiparallel beta-sheet (21%), beta-turn (22%) and unordered conformations (41%). An analysis of the mutant peptide species also predicted elements of alpha-helix (8%), antiparallel beta-sheet (28%), beta-turn (22%) and unordered conformations (40%). Thus, a small difference in the stabilized secondary structural conformations of the two sets of peptide species might partly explain their differential binding affinities for retinoblastoma protein, but it is likely that electrostatic charge differences resulting from the glutamic acid to lysine substitution play a dominant role.

Determination of subunit contact-associated epitopes of the beta-subunit of human follicle-stimulating hormone

Three different experimental approaches were used to assess the regions on the beta-subunit of human FSH (hFSH beta) that may be altered or masked by its association with the alpha-subunit of hFSH (hFSH alpha) in the heterodimeric hFSH molecule. In a direct approach, we tested whether synthetic peptides corresponding to hFSH beta sequences 1-20, 16-36, 33-53, 49-67, 66-85, 81-100, and 98-111 inhibited association of hFSH alpha and hFSH beta in an enzyme-linked immunosorbent assay. Synthetic peptides-(81-100), -(98-111), and -(66-85) caused greater than 50% inhibition of subunit association, whereas other peptides showed 26% or less inhibition. These data suggested that the C-terminal sequences of hFSH beta, particularly 81-100, are at a subunit interface with hFSH alpha in heterodimeric hFSH. In another approach we reasoned that antibodies with a higher affinity for free hFSH beta than for heterodimeric hFSH bind to epitopes on hFSH beta that are masked or altered by hFSH alpha subunit. To test this hypothesis, epitopes of hFSH beta were mapped using synthetic peptides of hFSH beta sequences, three monoclonal antibodies (3G3, 4D5, and 4G8), and a polyclonal antiserum (NIDDK anti-hFSH beta). Compared to 3G3 all the other antibodies exhibited minimal reactivity with hFSH, but bound strongly to hFSH beta. The epitope-mapping data with both 4D5 and NIDDK anti-hFSH beta identified peptide 81-100, which was not recognized by 3G3. The epitope map with 4G8 identified the same three peptides as with 3G3. However, in the case of 4G8 its reactivity with peptide 33-53 was the least, whereas it was ranked first for 3G3. Since both 3G3 and 4G8 had an identical affinity for hFSH beta, it was hypothesized that sequences in peptide 33-53 may be altered or masked by hFSH alpha. To test this, we determined the specificity of anti-hFSH beta-(33-53) peptide antiserum for hFSH beta and hFSH in an enzyme-linked immunosorbent assay. The antipeptide antiserum bound strongly to free hFSH beta and weakly to hFSH, suggesting that part of the sequence in peptide-(33-53) was masked or altered by association with hFSH alpha in heterodimeric hFSH. Taken together, the subunit association studies, the epitope-mapping data, and the specificity of anti-hFSH beta-(33-53) peptide antiserum have suggested that sequences in peptide-(81-100) and -(33-53) are masked or conformationally altered by hFSH alpha in heterodimeric hFSH.

Epitope mapping of human follicle stimulating hormone-alpha using monoclonal antibody 3A identifies a potential receptor binding sequence

Five monoclonal antibodies (mabs) were generated (3A, 4B, 5F, 2E, 1E) by immunizing BALB/c mice with human (h) FSH. The mabs were used to relate antigenic structures (epitopes) to function (receptor binding). All five mabs could immunoneutralize (inhibit binding to receptor) hFSH and could be placed into two groups based on potency (degree of neutralization). Group I mabs (5F, 2E, 1E) were less potent than group II mabs (3A, 4B) even though group I mabs had a 2-fold higher average affinity constant than group II. Those data suggested that group II mabs recognize an epitope near or in the receptor binding site of hFSH. Immunoradiometric epitope cross-matching demonstrated that group I and group II mabs recognize different epitopes. Further characterization of 5F and 3A (representative of group I and group II, respectively) utilized an enzyme-linked immunosorbent assay (ELISA) and a RIA. In the ELISA, both mabs bound hFSH and hFSH alpha but not hFSH beta. In the RIA, 3A bound [125I]hFSH and [125I]hFSH alpha but not [125I]hFSH beta. In contrast, 5F bound only [125I]hFSH. hFSH effectively competed with [125I]hFSH for 5F and 3A. In contrast, hFSH alpha competed with [125I]hFSH for 5F but not for 3A even though 3A could bind hFSH alpha in the ELISA and the RIA. These results suggest that 3A and 5F recognize different epitopes. The epitope recognized by 3A is unique in that its conformation appears to be dependent on association with hFSH beta. Since 3A was a more potent inhibitor of receptor binding than 5F, its epitope specificity was characterized further by epitope mapping. This was accomplished utilizing a peptide ELISA and by affinity chromatography. The results from epitope mapping demonstrated that 3A recognizes sequences 61-78 and 73-92 with binding to 73-92 being 4-fold greater than to 61-78. Thus, the epitope comprised of sequence 73-92 (and to a lesser extent 61-78) appears to be important for receptor binding.

Identification of subunit contact sites on the alpha-subunit of lutropin

Peptides corresponding to the entire sequence of the alpha-subunit of the human glycoprotein hormones were synthesized by using standard solid-phase procedures. Purified peptides were incubated in the presence of alpha- and beta-subunits of bovine lutropin, and subunit recombination was monitored by difference spectroscopy, reverse-phase high-pressure liquid chromatography, and gel filtration chromatography. Although the binding of alpha-peptides to either subunit could not be detected by these techniques, it was possible to demonstrate that some peptides could inhibit the recombination of alpha- and beta-subunits. Specifically, alpha-peptide 33-58 allowed only 0-11% of subunit recombination in 24 h (38-56% after 48 h), while alpha-peptide 51-65 allowed 10-60% of subunits to recombine in 24 h (65-94% in 48 h). Peptides 1-15, 11-27, 22-39, 61-78, and 73-92 of the alpha-subunit could not inhibit subunit recombination at any time or at any concentration tested. The data suggest that at least a portion of the alpha-subunit contact site has been identified, and results are discussed in terms of protein structure assessment tools.

Topographic analysis of the alpha-subunit of human follicle-stimulating hormone using site-specific antipeptide antisera

Structure-function investigations were undertaken to increase understanding of the surface topology of the alpha-subunit of human FSH (hFSH). The objectives were to determine which sequences of the alpha-subunit of hFSH are surface-oriented (exposed to antibody) and to identify which of these surface-oriented sequences are in contact with the beta-subunit of hFSH in the alpha/beta heterodimer. Seven overlapping synthetic peptides spanning the primary structure of hFSH alpha were used for immunizing rabbits to generate site-specific antipeptide antisera. The antisera were characterized with respect to their reactivity to the seven synthetic peptides, as well as hFSH, hFSH alpha, hFSH beta, and hFSH alpha r/a (reduced and alkylated), using an enzyme-linked immunosorbent assay. All of the peptides successfully generated antipeptide antisera with titers that range from 1:1,600 to 1:80,000. Anti-1-15 bound exclusively to hFSH alpha. Anti-11-27 and anti-33-58 bound to hFSH alpha to a much greater extent than to hFSH. In contrast, anti-73-92 had only slightly higher binding to hFSH alpha than to hFSH. Anti-22-39, anti-51-65, and anti-61-78 all failed to bind to either hFSH or hFSH alpha. With the exception of anti-22-39, all of the remaining antisera bound to hFSH alpha r/a. None of the antisera bound to hFSH beta. These data strongly suggest the following. Sequences 1-15, 11-27, and 33-58 contain residues that are masked by hFSH beta and are thus in or near the alpha/beta-subunit interface. In addition, sequences 11-27 and 33-58 contain other distinct residues that are surface-oriented in the hFSH heterodimer. In contrast, sequence 73-92 appears to be surface-oriented in the hFSH heterodimer. Lastly, sequences 51-65 and 61-78 appear to be buried within the native alpha-subunit and, thus, are unable to interact with antibodies. These results agree with and extend previous findings and will prove useful to those currently investigating the surface topology and structure-function relationships of the glycoprotein hormones.

Mapping of an assembled epitope of human follicle-stimulating hormone-beta utilizing monoclonal antibodies, synthetic peptides, and hormone-receptor inhibition

Monoclonal antibodies (mabs) to human (h) FSH were utilized to probe epitopes of the beta-subunit of hFSH (hFSH beta). These mabs had an average approximate affinity constant (Ka) of 10(8) M-1 for hFSH beta and 10(7) M-1 for heterodimeric hFSH. Hormone specificity of mabs for hFSH beta was demonstrated by a lack of cross-reactivity with hCG alpha, FSH alpha, or LH alpha. Epitope specificity of each mab was initially assessed by determining whether solid phase mab could bind to [125I]hFSH already bound to mabs in liquid phase. In addition, it was determined whether [125I]mab could bind to hFSH already bound to solid-phase mabs. Both epitope cross-matching protocols indicated that all mabs bound to the same epitopes on hFSH beta. Next, synthetic peptides corresponding to the sequence of hFSH beta were used in an enzyme-linked immunosorbent assay to map this epitope. All mabs bound to peptides 7-19, 1-20, 33-53 and 66-85 but did not bind or bound weakly to peptides 81-100, 95-103, and 103-110. Titration experiments were performed using different concentrations of peptide (0.3-41 nmol) and one mab 3G3 (500 ng-25 ng) in the enzyme-linked immunosorbent assay. The product of the lowest mass of both peptide and antibody which gave a positive result was used to rank the peptides for their binding with mab 3G3. Peptides were ranked in the following descending order of potency: 33-53, 49-67, 66-85 much greater than 16-36, 1-20, 95-103, 52-65, 81-100, and 103-110. Ability of the mabs to inhibit binding of [125I]hFSH to bovine testis membrane receptor (Rec) was also studied. When [125I]hFSH was preincubated with increments of each mab for 2 h at 25 C before adding Rec with further incubation for 16 h, all mabs inhibited [125I]hFSH binding to Rec. The data suggest that most of the hFSH beta molecule has a conformation enabling all antibody recognizable regions to be in close proximity to each other. The present study provides evidence for an assembled epitope comprising in part, amino acids 33-53, which has been previously shown to be involved in receptor binding. Peptide sequences 49-67 and 66-85 are neighboring sequences in this assembled epitope which contains the determinants for receptor binding.

Defining minimal requirements for antibody production to peptide antigens

The role that individual determinants play in modulating the immune response of an organism to a pathogen is often obscured because of the complexity of the pathogen. In order to gain a better appreciation of the role of individual determinants in the immune response, a pathogen may be dissociated into smaller components, for example peptides representing specific epitopes. These isolated components are often poorly immunogenic and historically have required the use of adjuvants to stimulate antibody production. This report defines the minimal essential requirements for antibody production to a peptide in this system. These are the ability to stimulate both B- and T-helper lymphocytes, anchorage in a phospholipid complex and multivalency within the complex. When these conditions are met, no additional adjuvants are necessary. This procedure has allowed us to identify three distinct T-helper cell epitopes from HIV gp160. In addition, this information has been used to produce a simple, totally synthetic and highly immunogenic preparation for the production of antibodies to peptides.

Endothelin-like pulmonary vasoconstrictor peptide release by alpha-thrombin

The endothelial cells lining the vessel wall can modulate vasomotor tone by releasing vasoactive factors, such as endothelial-derived constricting factors. We observed that alpha-thrombin, but not catalytically inactivated alpha-thrombin, mediated the release of two pulmonary vasoconstrictor peptides into the venous effluent of guinea pig lungs. These peptides elicited a slow-onset, long-lasting pulmonary vasoconstriction similar to the effect of endothelin, an endothelial-derived 21-amino acid vasoconstrictor peptide previously isolated from cells in culture. One of the isolated peptides coelutes with endothelin upon reverse-phase HPLC with an acetonitrile gradient and has a molecular weight comparable to endothelin as determined by gel-permeation HPLC. The other vasoconstrictor peptide elutes earlier than endothelin on reverse-phase HPLC and exhibits a lower molecular weight. The studies show the release of endothelin-like pulmonary vasoconstrictor peptides in the intact lung by alpha-thrombin, a central regulatory enzyme in hemostasis.

Purification and chemical composition of a low molecular weight follicle-stimulating hormone binding inhibitor from porcine follicular fluid

Porcine follicular fluid contains several factors capable of inhibiting the binding, in vitro, of follicle-stimulating hormone (FSH) to receptor, including an agonist and an antagonist of FSH biological activity in vitro. FSH receptor-binding inhibitory activity (FSH-BI) was determined with assays using radioligand (125iodide-human FSH) receptor (calf-testes membrane); in vitro biological assays (cultured immature rat Sertoli cells) were used to determine antagonist/agonist activity. FSH antagonist activity is due to a low (less than 5000) molecular weight FSH-BI that is soluble in acidic acetone and insoluble in diethyl ether allowing preparative scale isolation. Additional purification was achieved by anion-exchange and reverse-phase high-performance liquid chromatography. Highly purified, biologically active FSH-BI contained the amino acids Ser, Gly, Arg, Thr, Ala, Pro, Val, and Lys; hexoses (phenol-sulfuric acid-positive reaction); and ethanolamine. Thus, this FSH antagonist appears to be a complex glycopeptide--possibly derived from membrane components, as suggested by the presence of ethanolamine and carbohydrate residues. Porcine follicular fluid, therefore, contains a low molecular weight FSH antagonist that, along with the high molecular weight FSH agonist previously identified, may regulate gonadal responsiveness to FSH through interactions with the FSH receptor.

Cleavage of a model peptide at its glycine residue by alkaline mercuric oxycyanide

Treatment of peptides with excess HgO in the presence of alkaline cyanide leads to cleavage of the peptides at glycine residues. The reaction appears to involve both C- and N-mercuration with subsequent release of 2 mol mercury per mol of glycine. An intermediate glyoxylic acid residue in Schiff base linkage is postulated. Treatment of the heptapeptide Phe-Ala-Lys-Gly-Leu-Asp-Val with alkaline HgO and KCN for 6 h at 25 degrees resulted in greater than 90% cleavage, and the resultant reaction products were separated by reverse phase chromatography and identified by amino acid analysis. N-terminal products were approximately equimolar Phe-Ala-Lys, Phe-Ala-Lys-Gly, and Phe-Ala-Lys-amide. C-terminal products were predominantly Leu-Asp-Val (63%), plus Gly-Leu-Asp-Val (9%), and oxalyl-Leu-Asp-Val (8%). This method may be useful for cleavage of peptides or proteins containing glycine residues.

A proposed structural model of endothelin

We have assessed the hydropathic nature, secondary structure, and flexibility of endothelin and report a testable model for the structure of this 21-amino acid vasoconstrictor peptide. The proposed model consists of a rigid (disulfide bonded), hydrophilic amino-terminal half of the molecule with two turns, and an extended hydrophobic sheet structure comprising the C-terminal half of the molecule. The extended structure may be stabilized by intermolecular hydrogen bonding, leading to dimers or higher order aggregates, and is hydrophobic enough to partition into an organic solvent such as ether, as long as the molecule can keep its hydrophilic amino terminus in an aqueous phase. To test this model, partitioning measurements were made on bioactive synthetic endothelin. It was found to partition at the interface of an ether/water system in a concentration- and surface area-dependent manner. A further indication as to the amphipathic nature of the molecule is that microcrystals of synthetic endothelin tend to grow at an organic-aqueous interface in a two-phase system.

A calorimetric analysis of human plasma fibronectin: effects of heparin binding on domain structure

Fibronectin domain structure, as influenced by interaction with heparin, calcium, or chondroitin sulfate C, was analyzed by differential scanning calorimetry. A complex thermal denaturation transition was observed with a large sharp endotherm at 63 degrees C, a broad endotherm between 70 and 80 degrees C, and an exotherm at 80-90 degrees C. Analysis of the denaturation profiles revealed the existence of four thermal transitions, 59.1, 62.2, 67.3, and 74.3 degrees C, and an exotherm at 83.9 degrees C. The calorimetric enthalpies of the four endotherms are 1146 +/- 259, 866 +/- 175, 1010 +/- 361, and 676 +/- 200 kcal/mol, respectively. In all cases, the calorimetric to van't Hoff enthalpy ratio was greater than 1.0. Computer analysis of the primary structure of fibronectin revealed 29 +/- 8% homology among the type I homology units and 28 +/- 7% homology among type III homology units, suggesting that different structural domains could arise from the same homology type. This may explain why more thermal transitions are observed for fibronectin than there are homology types. Addition of heparin to fibronectin in varying molar ratios, i.e., 10:1 to 30:1, resulted in a larger calorimetric enthalpy for the first type of structural domain (Tm = 59.1 degrees C) of fibronectin. At higher heparin to fibronectin ratios (40:1 or 75:1), the enthalpy of this domain decreased, while the others remained unchanged. In the presence of 5 mM calcium chloride, fibronectin thermal denaturation occurred at lower temperatures and was associated with precipitation of fibronectin.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermodynamics of follitropin binding to solubilized calf testis receptor

Thermodynamic parameters of follitropin binding to solubilized testicular receptors were measured in order to assess the forces involved in the binding reaction. Reversibility of follitropin binding to solubilized receptor decreased only 20% over the temperature range 4-24 degrees C, whereas earlier studies indicated reversibility of binding to membrane-bound receptor decreased by more than 40% over the same range [Anderson, T. T., Curatolo, L. M., & Reichert, L. E., Jr. (1983) Mol. Cell. Endocrinol. 33, 37-52]. Thermodynamic analysis of follitropin binding to solubilized receptors showed that the hydrophobic effect was important in the binding reaction. The mean values, at 25 degrees C, for delta H and delta S were -31.8 kcal/mol and -66.0 cal mol-1 K-1, respectively, and delta Cp was -3.0 kcal mol-1 K-1. This is an unusually large heat capacity for protein-protein association reactions, indicating an enhanced role for the hydrophobic effect with the solubilized (compared to membrane-bound) receptor. Since glycerol was necessary to stabilize the solubilized receptor, we determined whether glycerol affected the thermodynamic parameters measured for the binding reaction. Control experiments, performed with membrane-bound receptor in the presence or absence of glycerol, indicated that delta Cp actually decreased upon addition of glycerol (-0.8 kcal mol-1 K-1 in the presence of glycerol compared to -2.3 kcal mol-1 K-1 in the absence of glycerol). Thus, the large negative delta Cp observed for the soluble receptor was a result of its removal from the membrane and was not due to the presence of glycerol.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of iodine-125-labeled human follitropin binding to testicular receptor by epidermal growth factor and synthetic peptides

Two tetrapeptide sequence homologies between mouse epidermal growth factor precursor (mEGFP) and human follitropin (FSH) were revealed by a computer program that identifies identical residues among polypeptide sequences. The two tetrapeptides, Lys-Thr-Cys-Thr (KTCT) and Thr-Arg-Asp-Leu (TRDL), are present in the hormone-specific beta subunit of FSH from all species studied. These tetrapeptides are not present in the alpha subunit, which is common to all pituitary glycoprotein hormones. Both tetrapeptides are also found in mEGFP, and one tetrapeptide, TRDL, is located within the 53-residue form of mEGF purified from mouse submaxillary glands. Computer-generated hydropathy profiles predicted that both tetrapeptides are located in hydrophilic portions of the FSH beta subunit and that TRDL is in a hydrophilic portion of commercially available mEGF. Therefore, the tetrapeptides might be accessible to receptor binding sites for FSH. We report that mEGF inhibits binding of 125I-labeled human FSH to receptors in testis by 50% (I50) at a concentration of 1.8 X 10(-5) M. No binding inhibition was observed by GnRH or arginine-vasopressin at 10(-4) M, neither of which contain the tetrapeptide sequences. FSH beta subunit, which contains both tetrapeptides, also inhibited binding (I50 = 9 X 10(-8) M) of 125I-labeled human FSH to testis receptor. Thus, it appears that FSH beta subunit and mEGF are capable of inhibiting binding of FSH to testicular FSH receptors, presumably through interactions that include the homologous tetrapeptides. This presumption was supported by the observation that the synthetic tetrapeptides (KTCT or TRDL) were also active in inhibiting binding of 125I-labeled human FSH to testis receptor.

Prediction of antigenic sites in follicle-stimulating hormones: difference profiles enhance antigenicity prediction methods

A method of predicting antigenic sites in proteins using only the information provided by primary structure has been reported. The basis of this method is that hydrophilic regions of a polypeptide should contain antigenic sites, since hydrophobic sites should be buried and inaccessible. However, if the antigen is homologous with a naturally occurring polypeptide in the immunized animal, the immune system may not recognize the predicted antigenic site as non-self. Therefore, a modification of the prediction scheme has been developed which can simultaneously examine a protein and its homolog. This method would predict as epitopes those sites that are hydrophilic (accessible) and unrelated to the host homolog (non-self). A computer program has been developed to compare polypeptides and generate difference profiles for antigenic sites. The method has been used to distinguish the antigenic determinants of ovine and human FSH.

Analysis of computer-generated hydropathy profiles for human glycoprotein and lactogenic hormones

Analysis of hydrophobic and hydrophilic regions of human lactogenic (PRL, GH, and placental lactogen) and glycoprotein hormones (FSH, LH, TSH, and hCG) by the method of Kyte and Doolittle has been performed. A BASIC program, developed for the IBM personal computer, produces graphical and tabular results. The net hydropathy value, a new parameter based on the Kyte and Doolittle analysis which may be useful for comparing various polypeptides, was developed. This value is correlated with physical properties, such as solubility of the glycoprotein hormones. New (more hydrophilic) indices were assigned for glycosylated asparagine, serine, and threonine residues, and slightly more hydrophilic indices were assigned for half-cystines found in disulfide bonds. The results indicate that the so-called determinant loop of Ward and Moore is hydrophilic (accessible) and suggest that regions on either side of this loop should also be considered as potential effectors of hormone specificity. It is suggested that binding (protein-protein interaction) sites tend to be modestly hydrophilic, but also contain residues that could interact through the hydrophobic effect.

Activity of alkylated prolactin and human growth hormone in receptor and cell assays

The disulfide bonds of two lactogenic hormones, ovine prolactin (oPRL) and human growth hormone (hGH), were reduced with dithiothreitol under denaturing conditions and alkylated with iodoacetic acid. The modified hormones were assayed for their ability to bind the plasma membrane-bound receptor for lactogenic hormone found in the rabbit mammary gland. S-Carboxymethylated ovine prolactin (SCM-oPRL) with all six cysteine residues modified had a nearly 300-fold decrease in binding as compared to native oPRL in a competitive binding assay using [125I]ovine prolactin. The S-carboxymethylated human growth hormone (SCM-hGH) had all four of its cysteine residues modified. It showed only a slightly reduced ability to bind the rabbit mammary gland prolactin receptor in a competitive binding assay with [125I]ovine prolactin. The two modified hormones were assayed for their ability to stimulate proliferation of the lactogen-dependent Nb 2 lymphoma cell line. SCM-oPRL required concentrations greater than 1 X 10(5) that of native oPRL to stimulate 50% of the maximum cell growth. SCM-hGH retained a significant amount of its ability to stimulate the Nb 2 lymphoma cells.

Correlation of B coefficient of viscosity for monovalent salts with effects on binding of human follitropin to receptor

Various monovalent salts have been shown to inhibit the binding of radioiodinated human follitropin [( 125I]hFSH) to receptors present in calf testis. We examined the effects of salts on the affinity constant (Ka) and number of binding sites (R0) in the FSH-calf testis receptor system. A constant amount of [125I]hFSH and increasing amounts of unlabeled hFSH were allowed to bind to a constant amount of receptor in the absence or presence (0.10 M) of halides, nitrates or acetates of the alkali ions at 20.0 degrees C. There was an appreciable variation of the affinity constant depending on the salt being used, but there was no change in receptor number (R0). Within an alkali cation series (e.g. NaCH3CO2, NaF, NaCl, NaBr, NaNO3, NaI or a similar potassium series) the affinity decreased with decreasing B coefficient of viscosity. Within a halide series (e.g. LiCl, NaCl, KCl, RbCl or the similar bromide series) the minimum value of the affinity constant occurred with the Na+ salt. The special inhibitory potency of Na+ may suggest that it has a unique interaction with calf testis FSH receptor or with follitropin. The studies indicate a correlation between the B coefficient of viscosity and the ability of salts to inhibit binding of [125I]hFSH to receptor.

Follitropin binding to receptors in testis: studies on the reversibility and thermodynamics of the reaction

Studies were designed to assess the extent to which binding of follitropin (FSH) to its receptor is a reversible process. [125I]hFSH was allowed to interact with membrane receptors from calf testis for 2 h at various temperatures by which time significant specific binding of [125I]hFSH had taken place in all instances. Unlabeled FSH was then added (delayed addition) and the amount of [125I]hFSH remaining bound was monitored as a function of time. In order to assess reversibility, [125I]hFSH binding in the latter samples was compared to that occurring when unlabeled FSH had not been added or when unlabeled FSH was present from the start of the incubation. Binding was essentially fully reversible at 4 degrees C, but reversibility decreased with increasing temperature. Reversibility of FSH binding decreased markedly at temperatures greater than 26 degrees C and was considered irreversible at temperatures above 30 degrees C. At 4 degrees C essentially full reversibility (greater than 90%) was observed when the unlabeled hormone was added after 7 h of incubation, but decreased when added after 12 h. At warmer temperatures (22 or 30 degrees C) 'there was a progressive decrease in reversibility as the time of delay before addition of unlabeled hormone was lengthened. By determining the affinity constant (in separate experiments) at various temperatures, a thermodynamic analysis was possible. This analysis was restricted to the temperature range 4-26 degrees C in order to minimize complications arising from irreversible binding. The reaction was endothermal at low temperatures (T less than 12.5 degrees C) and exothermal at higher temperatures (T greater than 12.5 degrees C) and was associated with a decrease in heat capacity of 1800 cal [mol deg]-1 at 25 degrees C. The results are consistent with the concept that the hydrophobic effect plays an important role in FSH binding, but that the reaction is complex and may be composed of more than one step.

Physical studies on the rabbit hepatic galactoside-binding protein. Effects of calcium and ligands

The rabbit hepatic galactoside lectin exhibited noncooperative binding of asialo-orosomucoid in four different nonionic detergents, Triton X-100, octyl glucoside, cetyl eicosaoxyethyleneglycol monoether (Brij 58), and dodecyl octaoxyethyleneglycol monoether (C12E8). The Brij 58-solubilized lectin chromatographed as a single peak upon gel filtration on Sepharose 4B and the molecular weight was determined to be 234,000 in the absence of calcium by sedimentation equilibrium ultracentrifugation. When calcium or calcium plus ligand was added, the molecular weight of the lectin increased to 612,000. A pronounced (15%) decrease in the intrinsic fluorescence of the galactoside lectin was observed upon addition of calcium. Based upon changes in fluorescence, the equilibrium dissociation constant for calcium and binding protein was 1.5 x 10(-3) M. No major changes were detected upon calcium addition by ultraviolet absorption or circular dichroism spectroscopy, nor were there any changes when a ligand such as lactose was added. The number of calcium ions bound, as determined by ultrafiltration, was 3.3 Ca2+/polypeptide chain in the absence of other divalent metals and 1.92 Ca2+/chain in the presence of 100 mM Mg2+. The equilibrium dissociation constant determined in this manner for Ca2+ was 3.5 x 10(-4) M.

Calorimetric study of the rabbit hepatic galactoside binding protein: effects of calcium and ligands

Differential scanning calorimetry has been used to examine the thermal denaturation of rabbit hepatic galactoside binding protein. In the absence of Ca2+ or ligands, the inactive binding protein shows a single transition with a Tm of 46 +/- 0.5 degrees C and an enthalpy of denaturation of 0.891 cal g-1. In the presence of 20 mM CaCl2, the active binding protein has a single transition with a Tm of 61 degrees C and an enthalpy of denaturation of 2.67 cal g-1, indicating that Ca2+ markedly stabilizes the protein toward thermal denaturation. The Tm values of the binding protein--Ca2+ complexes with asialoorosomucoid or lactose are 64 and 63 degrees C, respectively. The enthalpy of denaturation in the presence of 20 mM lactose is 3.39 cal g-1, indicating that an additional stabilization (approximately 27%) toward denaturation is provided by binding of specific ligands. Furthermore, the differences in the shape of the denaturation profiles in the presence and absence of ligands suggest that ligand binding influences the denaturation process. Calcium binding, however, stabilizes the galactoside binding protein to thermal denaturation to a greater extent than does ligand binding. Thermal denaturation transitions attributable to the A or the B subunits of the binding protein are not observed, suggesting that the two subunits may be structurally similar.

Reaction of the histidines of prolactin with ethoxyformic anhydride. A binding site modification

The seven histidines of bovine prolactin were modified with ethoxyformic anhydride and two classes of reactivity were apparent: 5 histidines were in the more reactive class (k = 0.097 min-1) and 2 histidines were less reactive (k = 0.011 min-1). The activity of the modified prolactins was determined by measuring their ability to bind to prolactin receptors from rabbit mammary glands. This assay showed that prolactin was fully active when 0 to 5 histidines were modified. If all 7 residues were modified, the hormone was unable to bind to its receptor. Circular dichroism studies indicated no significant differences in conformation for prolactins which had 2 to 7 histidines modified. Modification of human growth hormone and human placental lactogen with ethoxyformic anhydride resulted in a loss of the ability of these lactogenic hormones to bind to the prolactin receptor. For all three hormones, essentially full activity was recovered when the modifying group was removed by treatment with hydroxylamine. Sequence comparisons indicate that only 2 of the 3 growth hormone histidines and 2 of 7 placental lactogen histidines were homologous with histidines in bovine prolactin and that, in each case, they correspond to His-27 and His-30 in bovine prolactin. It is postulated that these residues serve to identify a portion of the binding domain of bovine prolactin.